1. Field of the Invention
The present invention relates to a process for producing a polyurethane elastomer having excellent physical properties by a two liquid mixing method at a lower temperature. More particularly, it relates to a process for producing a polyurethane elastomer having excellent physical properties by mixing at an ambient temperature and fast curing at a lower temperature by using a hardener composition containing a polyamine, a polyol and a catalyst and an isocyanate composition containing an organic diisocyanate monomer and an isocyanate prepolymer.
2. Description of the Prior Arts
Heretofore, polyurethane elastomers have excellent characteristics such as high strength, elasticity, wearing resistance and oil resistance, and accordingly polyurethane elastomers have been widely used for various fields such as parts for cars, parts for industrial instruments, and parts for sport goods.
As a process for producing polyurethane elastomers, it has been proposed and widely applied that a prepolymer having terminal isocyanate groups be prepared by reacting a long chain diol with an excess of an aromatic diisocyanate and deaerated by heating it at about 100.degree. C. and admixing with a molten hardener such as an aromatic diamine, (such as 4,4'-methylenebis(2-chloroaniline) having a melting point of 95.degree. C. referred to a MOCA which is usually used) heating at about 100.degree. to 110.degree. C. and curing at 100.degree. C., as a hot-melt molding method by a prepolymer method.
Thus, the prepolymer method for the hot-melt molding method has many following disadvantages which should be resolved.
Firstly, the solid hardener (such as MOCA) should be heat-melted and the prepolymer should be heated so as to prevent a solidification of the hardener and to make easy the deaeration whereby the aromatic diamine and the aromatic diisocyanate in the prepolymer are disadvantageously vaporized. These compounds are toxic to humans. An operator may breathe in the fine dust or varporized gas of the aromatic diamine. In the present stage, said breathing has not been successfully prevented.
Secondly, the aromatic diamine is used after heat-melting whereby physical properties of the resulting polyurethane elastomer are deteriorated and a hue is remarkably changed by the heating to cause a lower value polyurethane elastomer goods. Moreover, the terminal isocyanate groups are activated by heating the prepolymer to react with urethane groups of the prepolymer. This causes the allophanate crosslinking to increase the viscosity and to deteriorate physical properties of the polyurethane elastomer.
Thirdly, it is difficult to maintain stable conditions for the process by the increase of the viscosity of the prepolymer as mentioned in the Second problem. Therefore, it is quite difficult to attain a mass production by an automatic continuous mixing method.
Fourthly, the mixing, the hardening and the curing are carried out at high temperature whereby a large quantity of heat energy is consumed to cause high cost for the preparation. These are disadvantages found in the conventional method.
Various studies for improving these problems have been presented. For example, a polyamine is dissolved in a solvent of polyethyleneglycol dibenzoate or polypropyleneglycol, and is mixed with a prepolymer having terminal isocyanate groups to harden the mixture at an ambient temperature. Thus, in accordance with this process, the solvent of polyethyleneglycol dibenzoate or polypropyleneglycol is not reactive to remain in the resulting polyurethane elastomer as a plasticizer whereby the physical properties of the urethane elastomer is disadvantageously lowered. It has been known to produce a polyurethane elastomer by the one shot method of reacting a long chain diol, an organic diisocyanate and a specific tetrahalogenated aromatic diamine in one step (Japanese Examined Patent Publication 45316/1976). In this process, the aromatic diamine is dissolved in the long chain diol, whereby there is no trouble concerning breathing of fine dust or vapor of the aromatic diamine. Moreover, a polyurethane elastomer having high hardeners and excellent physical properties can be advantageously produced. However, the specific halogenated aromatic diamine should be used as the hardener. This is serious problem.
The inventors have studied to develop the improved process for producing a polyurethane elastomer by overcoming the disadvantages and the problems found in the conventional processes.